Shielded amplifier reversing switch



May 1l, 1943. R. c. CURTIS SHIELDED AMPLIFIER REVERSING SWITCH Filed Dec. 27, 1941 Patented May 11, 1943 UNITED STATES., PATENT OFFICE SBIELDED AMPLIFIER. REVEBSING SWITCH Roger C. Curtis, New Haven, Conn., signor to Dictaphone Corporation, New York, N. Y., a corporation of New York Application December 27, 1941, Serial No. 424,578

13 Claims. I (Cl. 200-1) quired in such a machine because o! the danger of feedback in the ampliner. The input and the output circuits oi the amplier both must be connected to contact points in the switch structure, and, even though all of the connecting circuits are carefully protected by grounded shields, there is ordinarily suiilcient capacity between these contact points to produce a feedback phenomenon in the ampliiler by electrically interconnecting the input and output circuits.

frequency, low-impedance A. C. circuits because The diilicuities of this situation are even further oi' the eiIect oi the capacity between the various enhanced ii high impedance translating devices, contact elements of the switching arrangements. such, for example, as piezo-electric crystal de- This intercontact capacity is usually oi suillcient vices, are used in the microphone-loudspeaker value to cause a transfer of A. C. energy between and the recorder-reproducer units. contact elements of the switch, even though the 15 Accordingly, therefore, it is a further object oi contact elements are not physically connected. this invention to provide in an electric dictating It is an object of this invention to provide a machine a simple and inexpensive low-capacity simple and inexpensive switching arrangement switching arrangement for eii'ectively reversing in which the detrimental effect of such interthe amplifier connections when the machine is contact capacity is eliminated. changed from recording to reproducing condi- In electric dictating machines the problem of tion, and vice versa. providing a suitable switching arrangement for These and other apparent objects and .adinterconnecting various audi-frequency circuits vantages of this invention are attained by the is a particularly difficult one. In such machines, novel structure described in the following specifor example, it is customary to provide a single fication which may be more readily understood translating unit used both as a recording microby reference to the accompanying drawing in phone and as a reproducing loudspeaker, a single which: amplifier unit, and a single translating unit co- Figure 1 is a Schematic wiring diagram of the acting with a record to act both as a recording audi-frequency circuits oi an electric dictating head and as a reproducing pickup. When such machine incorporating a switching arrangement a machine is adjusted for recording, the microin accordance With the invention; phone unit is connectedtothe input of the ampli- Figure 2 is a side-elevation of a switching iler and the recording head is connected to the Structure 0I ihe type ShOWn digielnmtically in output or the amplifier. When the machine is Figure 1. with a portion of the shielding casing conditioned for reproducing the ampliiex concut away to show the construction of the switchnections are reversed, the microphone unit is dising elements; connected from the input of the amplier and Figure 3 is an end-elevation of the terminal connected to the amplliler output to act as a. DOrtiOn 0f the Switching Structure shown in loudspeaker, and the recording head is discon- 40 Figure 2; 8nd nected from the output of the ampliiler and con- Figure 4 iS a iep Plan View 0f the Switching nected to the amplifier input t0 act as a, phonostructure shown in Figures 2 and 3 with part graph pickup. To be commercially practical such of the shielding casing shown broken away. a machine must be arranged so that this ampli- Referring to Figure l, a preferred embodiment iler reversal can be accomplished by a simple oi' the invention is shown, in more or less diaswitching operation. If only direct currents or grammatic form for convenience in illustration low-frequency low-impedance alternating curand description, incorporated in an electrical rents were involved this switching operation dctating machine of the type described, ior excould be accomplished by providing an ordinary ample. in U. S. Patent No. 2,143,819 to Leland D. double-pole double-throw switch to reverse the Norton The mechanical structure of such a connections of the amplier. However, due to dictating machine is described in the Norton intercontact capacity effects, a simple D. C. patent and details of its construction are not switching arrangement cannot be used for high necessary for a complete understanding of the gain audio-frequency circuits of the type represent invention. A dictating machine oi this type for both recording and reproducing speech essentially comprises a microphone unit i2, which also serves as a loudspeaker when reproducing, a suitable ampliiier I4, a translating unit I6 capable of being selectively conditioned for recording or reproducing by means of a shift lever I and slidably mounted to traverse a record I8 driven by suitable driving means (not shown), and an amplier reversing switch, generally indicated at 20, for interchanging the connections of the amplifier for dictation or listening-back. The combination microphone-loudspeaker unit I2 and the combination recorder-reproducer translating unit I6 may be any suitable type of translating devices capable of such double-purpose operation, such, for example, as the piezo-electric crystal devices shown in the above-mentioned Norton patent. The switch mechanism 20 may be operated manually by control member 24, or it may be operated by a mechanical interconnection with the shift lever I5 mounted on the translating unit I6. In Figure 1 such a mechanical interconnection is shown diagrammatically. A dictatellsten shift lever 26, suitably mounted on the dictating machine, is connected by a member 20 to the switch control member 24 and by another member 30 to the record-reproduce conditioning lever I5 of the recorder-reproducer translating unit I6. A suitable mechanical arrangement for accomplishing this mechanical interconnection, which per se forms no part of the invention herein described, is shown in the above-mentioned U. S. Patent 2,143,819 to Norton.

The amplifier switch 20 is essentially a doublepole, double-throw switch, cross-connected to act as a reversing switch. It is connected to one side of the microphone-loudspeaker unit I2, the input and output circuits of amplifier I4, and the translating unit I6 to connect the microphone-loudspeaker unit I2 to the input of the amplier I4 and the output of the amplifier to the recorderreproducer unit I6 when the machine is conditioned for dictating, and to reverse the-amplifier connections so that its input is connected to the recorder-reproducer device I6 and its output is connected to the microphone-loudspeaker unit I 2 when the machine is conditioned for listeningback. The other terminals of the microphoneloudspeaker unit I2, the ampliiier input and output circuits, and the recorder-reproducer unit I6 are connected to an electrically grounded circuit 22, common to all of the apparatus. A switch structure such as would ordinarily be proposed for accomplishing such a reversing ef-y fect is shown in the above-mentioned Norton Patent 2,143,819. However, such a standard switching arrangement, especially when used to interchange the high impedance input and output terminations of an amplier of the type designed for eicient operation with high impedance translating devices such as piezo-electric crystals, has the disadvantage that its contacts, certain of which necessarily must be connected to the input and output circuits of the amplifier, must be so disposed as to be in capacitive relation to each other, thus capacitively coupling the amplifier Iinput and output. Such capacitive coupling between the input and output of an ampliiier permits energy feedback which causes the amplier to oscillate and renders it inoperative.

In accordance with the present invention, a simple and inexpensive switch mechanism is provided for accomplishing this ampliiier reversal which has an extremely low capacity between the various contacting elements of the switch structure due to the provision of an arrangement which automatically interposes electrically grounded shielding elements between the contacts of the switch when the contacts are opencircuited. Referring to Figures 2. 3, and 4, this simple and novel shielded reversing switch arrangement 20 includes two identical switching units, generally indicated at 34 and 36. Each of these switching units 34 and 36, comprises a stack of seven leaf-spring contact members 2, 4 6, I, 1, 5, and 3 of varying lengths suitably arranged in the order stated in parallel relationship in a usual switch-stack construction, insulated from each other by insulated spacing members 38 and clamped together and to a supporting bracket by bolts 42.

As shown in the drawing, each switch stack is arranged so that the central operating leaf-spring member I, the longest of the contact springs, is adapted to be selectively positioned in a lefthand or right-hand position by the control mem- 'ber 24. This spring I carries a double-faced contact member IOI, projecting on either side of the spring, which is moved into contact with contact points |04 or |05 mounted on the free ends'of springs 4 and 5, respectively, as operating spring I is moved to one or the other of its alternate positions. Leaf springs 2 and 6, and 1 and '3, all slightly shorter than springs 4 and 5 and positioned, respectively, on either side of springs 4 and 5, respectively carry contacts |02, |06, |01, and |03 on their free ends. Thus, as shown in the drawings, contacts |02 and |06 alternately contact spring 4 and contacts |01 and |03 alternately contact spring 5 as operating spring I is moved to one or the other of its alternate positions. Leaf-spring contact members 4 and 5 are normally biased inwardly toward operating spring so that they are normally in 'contact with contact points |06 and |01, respectively. The structure of switch stack 36, comprising leafspring contact members 2', 4', 6', I', l', 5', and 3 arranged in that order, is identical with that of stack 34.

The two switch stacks 34 and 36 are each surrounded by electro-static shielding casings 44 and 46 (shown in dotted lines in Figure 1), provided with apertures 46 and 50, respectively, to admit the control member 24 to move the operating springs I and I. These shielding members 44 and 46 are provided with tabs 52 and 54 extending between the sets of connection lugs on the ends of each of the switch stacks, as shown in Figure 4, so as to provide a shield between the connection terminals of the two switching units 34 and 36.

In the embodiment of the invention herein described, connections to switching arrangement 20 are made as shown in Figure 1. Contact I of switch element 34 is connected to one side of the amplifier input and contact I of element 36 is connected to one side of the ampliiier output. Contact 2 is connected to one side of the microphone-loudspeaker unit I2 and to contact 3'. Contact 2' is connected to one side of the recorder-reproducer translation unit |6 and to contact 3. Contacts 6, 1, 6', and 1 are all connected together and to the electrically grounded circuit 22 of the apparatus, as are the shielding members 44 and 46. As above mentioned, this electrically grounded circuit 22 also forms the circuit common to the microphone-loudspeakerunit I2, the amplifier I4, and the recorder-repro the circuit only when they are in contact with otherc'ontact members.

In operation, when the shift lever 26 is moved to ."dictate position, as shown in Figure l. leafspring contact member I is moved to its letthand position by control member 24, bringing contact in contact with contact point I 04 on contact spring 4. Contact spring 4 is then moved with contact spring I to the lett, moving spring 4 out of contact with contact point |06 and into contact with contact point |02 on contact spring 2. This connects ,microphone-loudspeaker unit I2 to the input of ampliiier I4. In a similar manner contact spring I' is moved to the lett, making a connection with contact 4' and moving contact 4 out of contact with contact 6 and into contact with contact 2', thus connecting the output of amplifier I4 to the recorder-reproducer unit I4. This motion oi' contact springs and I' to the left to connect ampliiler I4 in the circuit in the proper direction for recording, also permits contact springs 6 and 6 to assume their normal positions in contact with contact points |01 and |31' o n contact springs 1 and 1', respectively. This connects contact springs 6 and 6' to ground and thus serves to automatically interpose shielding elements, in the form of springs 6 and 6', be-

tween contact springs 3 and-|and 3' and I', respectively. Thus the input and the output circuits of amplier I4 are automatically shielded from each other, insofar as the switch contacts are concerned, thereby preventing feedbackr therebetween which would cause oscillations in mon electrically grounded circuit 22 (as indicated in the dotted lines of Figures 3 and 4), are brought out from .the sides of the switch-stack structures at points intermediate to the positions of connection terminals 20| and 20|', and connection terminals 202, 203, 202', and 203'. This disposition of the various connection terminals of the switching structures herein described,

places all of the electrically grounded contacts of the switching structures between the electrically *"high connection terminals and thus these the amplifier. 'In accordance with usual pracmoved to their right-hand positions, connectingL the recorder-reproducer unit I6 through contacts 3, 6, and I to the input of amplifier I4, and connecting the output oi' the amplifier through contacts I', 6', and 3' to the microphone-loudspeaker unit I2; while simultaneously contacts 4 and 4' move to their normal positions in contact with grounded contacts 6 and 6' to automatically interpose themselves as shields between contacts 2 and I, and 2' and I', respectively, thereby to shield the amplifier input circuit from the amplier output circuit as above described.

In order to enhance the shielding effect of contact members 4, 5, 4', and 5', they are preferably made of wider strip material than the remaining contact springs, as shown in the broken-away portion of Figure 4, so that they completely blanket the outer contacts 2, 3, 2 and 3', respectively, and nearly touch the sides of the enclosing shielding casings 44 and 46. y

The inter-circuit capacity of a switching structure, made in accordance with the present invention, can be further reduced by suitable disposition of the connection terminals attached to each of the leaf-spring contacts. Referring to Figures 2. 3, and 4, connection terminals or lugs 20| and 20| forming portions of control springs I and I. the center elements oi' the switching units 34 and 36, respectively, are brought out from the sides of the switch-stack structures at points near the contact ends oi' these structures, as may be most easily seen in Figure 4. Connection terminals 202, 203, 202', and 203', forming parts of contact springs 2, 3, 2', and 3', respectively, the outside grounded connection terminals act as eil'ective electro-static shields therebetween. To further aid this shielding eii'ect, it is also desirable t0 extend electrically-grounded leaf springs 8, 1, 6', and 1' beyond the clamped portion of the stack so that they project between connection .terminals 202 and 203, and 202' and 203', respectively,- so as to shield the connections made to those terminals.

It is apparent that a simple self-shielding switching arrangement, made in accordance with the present invention, is not limited to use in an electrical dictating machine and that it can be advantageously used in any electrical switching system in which it is necessary to interconnect various alternating current circuits, especially if such alternating current circuits are high impedance circuits carrying high frequency currents.

As many possible embodiments of the present invention may be made without departing trom the scope thereof, it is to be understood that al1 matter set forth or shown in the accompanying drawing is to be interpreted as illustrative only and not in a limiting sense.

Iclaim:

l. A self-shielding switch structure, comprising, in combination, first contact means, second contact means, conductive means associated with said contact means to open and close a circuit therebetween, said conductive means adapted in its open-circuit position to be interposed as a shield between said contact means, and means connecting said conductive means to ground when in its open-circuit position.

2. A self-shielding switch structure, comprising, in combination, first contact means, second contact means positioned adjacent said rst contact means but spaced therefrom, conductive means associated with said contact means movable to bridge the gap therebetween to open and close a circuit, and grounded means, said conductive means being movable out of said bridging position to a position between said contact means and in contact with said grounded means to act as a shield between said contact means when said circuit is open. y

3. A self-shielding switching structure, comprising, in combination, first contact means, second contact means, a conductive member interposed between said contact means, control means for relatively moving second contact means selectively in one direction to close a circuit between said contact means through said conductive member and in a second direction to open said circuit and ground said conductive member, whereby said 'conductive member serves as a shield between said contact means when the circuit therebetween is open.

4. A shielded switching arrangement, comprising, in combination, first contact means, second contact means, conductive means interposed between said rst and said second contact means, electrically grounded means adapted to contact said conductive means, mounting means movably supporting said first contact means and said conductive means for relative movement toward and away from said second contact means and said grounded means, and control means operatively associated with said rst contact means and said conductive means to selectively move them in one` direction relative to said second contact means and said grounded means to bring said first contact means into contact with said conductive means and to move the latter away from said grounded means into contact with said second contact means, and ina second direction to separate said first contact means, said conductive means, and said second contact means and return said conductive means to con-tact said grounded means, whereby said conductive means acts as a connector between said first contact means and said second contact means when said controlv means is operated in said first direction and as an electrically grounded shield therebetween when said control means is operated in said second direction.

5. A shielded switching arrangement, comprising, in combination, first contact means, second contact means, conductive means interposed between said rst and said second contact means, electrically grounded means adapted to contact said conductive means, mounting means movably supporting said first contact means and said conductive means for relative movement toward and away from said second contact means and said grounded means, and control means operatively associated with said rst contact means and said conductive means to selectively move them in one direction relative to said vsecond contact means and said grounded means to bring said first contact means into contact with said conductive meansand to move the latter away from said grounded means into contact with said second contact means, and in a second direction to separate said first contact means, said conductive means, and said second contact means and return said conductive means to contact said grounded means, whereby said conductive means acts as a connector between said first contact means and said second contact means when said control means is operated in said first direction and as an electrically grounded shield therebetween when said control means is operated in said second direction and shielding means surrounding all of said contact means and connected to said grounded means.

6. A shieldedswitching arrangement, comprislng, in combination, a first contact, a second contact, a member of conductive material interposed between said first and said second contacts, an electrically grounded contact adjacent said conductive member and adapted to contact the latter when it is moved toward said first contact, mounting means movably supporting said first contact and said conductive member for movement toward and away from said second contact, and control means operatively associated with said first contact to selectively move it in one f direction into contact with said conductive 'member to move the latter away from said grounded contact into contact with said second contact, and in a secondy direction to separate said first contact, said conductive member, and said second contact and return said conductive member to contact with said grounded contact, whereby said conductive member acts as a connector between said first contact and said second contact when said control means is operated in said first direction and as a grounded electro-static shield therebetween when said control means is operated in said second direction.

7. A shielded switching arrangement, comprising, in combination, a first leaf spring carrying a first contact point, a second leaf spring carrying a second contact point, a conductive member in the form of a third leaf spring interposed between said rst and said second springs and normally positioned out of contact therewith, a fourth leaf spring carrying an electrically grounded contact point interposed between said rst spring and said conductive member and normally biased to contact the latter, insulated mounting means movably supporting said springs in parallel relationship,v and control means operatively connected to said first leaf spring selectively to move said first contact point in one direction into contact. with said conductive member to pick up the latter and move it away from said grounded contact point into contact with said second contact point, and in a second direction to permit said first contact point, said conductive member, and said second point contact to separate and to restore the connection between said conductive member and said grounded contact point, whereby said conductive member acts as a connector between said first contact point and said second contact point when said control means is operated in said first direction and as a grounded electro-static shield therebetween when said control means is operated in said second direction.

8. A shielded switching device, comprising, in combination, f'lrst contact means, second contact means, third contact means positioned on the opposite side of said first contact means from said second contact means, first conductive means interposed between said first and said second contact means, second conductive means interposed between said rst and said third contact means, electrically grounded means adapted to contact each of said conductive means, mounting means movably supporting said rst contact means and both of said conductive means for relative movement toward and away from said second or said third contact means and said grounded means, and control means operatively associated with said first contact means and said conductive means to selectively move them in one direction relative said second contact means and said grounded means to bring said first contact an electrically grounded electro-static shield between said first contact means and said third contact means.

9. A shielded switching device, comprising, in combination, first contact means, second contact means, third contact means positioned on the vopposite side of said first contact means from said second contact means, first conductive means interposed between s'aid first and said second contact means, second conductive means interposed between said first and said third contact means, electrically grounded means adapted to -contact each of said conductive means, mounting means movably supporting said first contact means and both of said conductive means for relative movement toward and away from said second or said third contact means and 4said grounded means, and control means operatively associated with said first contact means and said conductive means to selectively move them in one direction relative said second contact means and said grounded means to bring said first contact means into contact with said first conductive means and to. move the latter away from said grounded means into contact with said second contact means, and simultaneously to separate 'said first contact means, said second conductive means, and said third contact means and permit said second conductive means to contact said grounded means, whereby said first conductive means acts as a connector between said first con- 'i tact means and said second contact means when said control means is operated in said first direction and said second conductive means acts as an electrically grounded electro-static shield between said first contact meansand said third contact means and shielding means surrounding all of said contact means and connected to said grounded means. v

10. A shielded switching device, comprising, in combination, a first leaf spring carrying a first contact point, a second leaf spring carrying a second contact point, a third leaf spring carrying a third contact point, a first conductive member in the form of a fourth leaf spring interposed between said first and said second springs and normally positioned out of contact therewith, a second conductive member in the form of a fifth leaf spring interposed between said first and said third springs and normally positioned out of contact therewith, a sixth leaf spring carrying an electrically grounded contact point interposed between said first spring and said first conductive member and normally biased to contact the latter, a seventh leaf spring carrying an electrically grounded contact point interposed between said i first and said second conductive member and normally biased to contact the latter, insulated mounting means movably supporting said springs in parallel relationship, and control means operatively connected to said first leaf spring selectively to move said first contact point in one direction into contact with said rst conductive mcmzer to pick up the latter and move it away from its corresponding grounded contact point into contact with said second contact point, and simultaneously to permit said first contact point, said second conductive member, and said third cfntact point to separate and to form a connection between said second conductive member and its corresponding grounded contact point, whereb" said first conductive member acts as a connector between said first contact point and said second contact point when said control means is operated in said first direction and said second conductive means acts as an electrically grounded electro-static shield between said leaf spring and said third contact point.

11. A shielded switching arrangement, comprising, in combination, a first leaf spring carrying a first contact point, a second leaf spring carrying a second contact point, a conductive member in the form of a third leaf spring interposed between said first and said second springs and normally positioned out of contact therewith, a fourth leafV spring carrying an electrically grounded contact point interposed between said first spring and said conductive member and normally biased to contact the latter, insulated mounting means movably supporting said springs in parallel relationship, control means operatively connected to said first leaf spring selectively to move said first contact point in one direction into contact with said conductive member to pick up the latter and move it away from said grounded contact point into contact with said second contact point, and in a second direction to permit said first contact point, said conductive member, and said second pointv contact to separate and to restore the connection between said conductive member and said grounded contact point, first terminal means connected to said first leaf spring and projecting fromv said mounting means at a first point, second terminal means connected to said second leaf spring and projecting from said mounting means at a second point, and third terminal means connected to said third leaf spring and projecting from said mounting means at a third point in between said first and second points, whereby said third terminal means acts as an effective electro-static shield between said first and second terminal means when said control means is operated in said second direction.

12. A shielded switching device, comprising, in combination, a first leaf spring carrying a first contact point, a second leaf spring carrying a second contact point, a third leaf spring carrying a third contact point, a first conductive member Y in the form of a fourth leaf spring interposed between said first and said second springs and normally positioned out of contact therewith, a second conductive member in the form of a fifth leaf spring interposed between said first and said third springs and normally positioned out of contact therewith, a sixth leaf spring carrying an electrically grounded contact point interposed between said first spring and said first conductive member and normally biased to contact the latter, a seventh leaf spring carrying an electrically grounded contact point interposed between said first and said second conductive member and normally biased to contact the latter, insulated mounting means movably supporting said springs in parallel relationship, control means operatively connected to said first leaf spring selectively to move said first contact point in one direction into contact with said first conductive member to pick up the latter and move it away from its corresponding grounded contact point into contact with said second contact point, and simultaneously to permit said first contact point, said second conductive member, and said thirdI contact point to separate and to form a connection between said second conductive member and its corresponding grounded contact point, first terminal means connected to said first leaf spring and projecting from said mounting means at a first point, second terminal means connected to said second leaf spring and projecting from said mounting means at a second point, third terminal means connected to said third leaf spring and projecting from said mounting means at a third point, fourth terminal means connected to said fourth leaf spring and projecting from said mounting means at a fourth point in between said first and second points, and iifth terminal means connected to said fifth leaf spring and projecting from said mounting means at a fth point in between said iirst and third points, whereby, said fourth terminal means acts as an effective electro-static shield between said first and second terminal means when said control means is operated in said second direction, and said ith terminal means acts as an effective electro-static shield between said first and said third terminal means when said control means is operated in said ilrst direction.

13. A shielded switching device, comprising, in combination, a iirst leaf spring carrying a rst contact point, a second leaf spring carrying a second contact point, a third leaf spring carrying a third contact point, a first conductive member in the form of a fourth leaf spring interposed between said first and said second springs and normally positioned out of contact therewith, a second conductive member in the form of a iifth leaf spring interposed between said iirst and said third springs and normally positioned out of contact therewith, a sixth leaf spring carrying an electrically grounded contact point interposed between said first spring and said first conductive member and normally biased to contact the latter, a seventh leaf spring carrying an electrically grounded contact point interposed between said first and said second conductive member and normally biased to contact the latter, insulated mounting means movably supporting said springs in parallel relationship, control means operatively connected to said first leaf spring selectively to move said first contact point in one direction into contact with said iirst conductive member to pick up the latter and move it away from its corresponding grounded contact point into contact with said second contact point, and simultaneously to permit said iirst contact point, said second conductive member, and said third con'- tact point to separate and to form a connection between said second conductive member audits corresponding grounded contact point, first terminal means connected to said first leaf spring and projecting from said mounting means at a first point, second terminal means connected to said second leaf spring and projecting from said mounting means at a second point, third termina1 means connected to said third leaf spring and projecting from said mounting means at a third point. fourth terminal means connected to said fourth leai' spring and projecting from said mounting means at a fourth point in between said first and second points, fifth terminal means connected to said ith leaf spring and projecting from said mounting means at a fifth point in between said first and third points, whereby,

said fourth terminal means acts as an eifective electro-static shield between said iirst and second terminal means when said control means is operated in said second direction, and said fifth terminal means acts as an effective electro-static shield between said first and said third terminal means when said control means is operated in said first direction, and conductive portions forming parts of said sixth and seventh leaf springs and projecting from the non-contacting ends of u said springs between said second and third terminal means to form an eiiective electro-static shield therebetween.

ROGER C. CURTIS.

' CERTIFICATE OF CORRECTION.

Y Patent No. 2,518,761. nay 11, 19M.

ROGER C. CURTIS.

It is hereby* certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 5, second column, line l, claim lO, before the words leaf spring" insert f1rst and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

signed and sealed this mth day of september, A. D. 19m.

Henry Van Arsdale, (Seal) Acting Commissioner of Patents.

CERTIFICATE 0F CORRECTION.

rPatent No. 2,518,761. May 11, 19H5.

ROGER C. CURTIS.

It is hereby' certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 5, second column, line l, claim 10, before the words flleaf Spring!! insert first and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

signed and sealed this mth day o'f september, A. D. 19LL5.

Henry Van Arsdale, (Seal) Acting Commissioner of Patents. 

